(1) N-ras activated oncogenes have been detected in human colon and liver carcinomas, as well as in one liver carcinoma-derived cell line (HepG2). The molecular structure and their transcriptional and translational products have been characterized. (2) Mammary carcinomas have been reproducibly induced in Buf/N female rats by injecting them with a single dose of nitroso-N-methylurea. All tumors obtained in that way contained a transforming H-ras-1 gene. Molecular cloning and characterization of one of these genes revealed that the 12th codon was GAA instead of GGA found in the normal allele, also cloned from untreated rats for comparison. The transforming gene encodes glutamic acid in place of glycine at position 12 of its p21 protein product. These findings demonstrated that chemical carcinogens can act on normal ras genes in a specific way and thus provides a useful model for studies aimed at determining the role of ras genes in the development of neoplasia. (3) A 27-kbp Eco RI DNA fragment, which seems to encompass the whole c-sis locus of a human glioblastoma (A172), has been molecularly cloned and partially characterized. (4) DNA sequences showing about 80% homology to retroviral v-fgr, v-src and v-sis oncogenes have been detected in the genome of the yeast Saccharomyces cerevisiae under relatively relaxed hybridization conditions. A 9 kbp Eco RI fragment enclosing the v-sis-related sequences has been cloned into the phage vector lambda L47.1, and its structure and function are being characterized.